linux-kernel - Re: [PATCH v7 6/6] writeback, cgroup: release dying cgwbs by switching attached inodes

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Date:   Fri, 4 Jun 2021 11:51:02 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Roman Gushchin <guro@...com>
Cc:     Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Dennis Zhou <dennis@...nel.org>,
        Dave Chinner <dchinner@...hat.com>, cgroups@...r.kernel.org
Subject: Re: [PATCH v7 6/6] writeback, cgroup: release dying cgwbs by
 switching attached inodes

Hello,

On Thu, Jun 03, 2021 at 06:31:59PM -0700, Roman Gushchin wrote:

> +bool cleanup_offline_cgwb(struct bdi_writeback *wb)
> +{
> +	struct inode_switch_wbs_context *isw;
> +	struct inode *inode;
> +	int nr;
> +	bool restart = false;
> +
> +	isw = kzalloc(sizeof(*isw) + WB_MAX_INODES_PER_ISW *
> +		      sizeof(struct inode *), GFP_KERNEL);
> +	if (!isw)
> +		return restart;
> +
> +	/* no need to call wb_get() here: bdi's root wb is not refcounted */
> +	isw->new_wb = &wb->bdi->wb;

Not a deal breaker but I wonder whether it'd be safer to migrate it to the
nearest live ancestor rather than directly to the root. As adaptive
migration isn't something guaranteed, there's some chance that this can
behave as escape-to-root path in pathological cases especially for inodes
which may be written to by multiple cgroups.

Thanks.

-- 
tejun